An X-ray CT apparatus irradiates an object with X-rays from an X-ray tube and collects X-rays, which are detected by an X-ray detector disposed at a position opposite to the X-ray tube and are transmitted through the object, as projection data. In this case, projection data from many directions is collected by rotating the X-ray tube and the X-ray detector opposite each other around the object, and a tomographic image of the object is reconstructed.
The projection data is collected at a position (hereinafter, referred to as a “view”) of a discrete X-ray tube. The collected projection data is called projection data at the view. The number of views per one revolution of the X-ray tube that rotates around the object is usually hundreds to thousands. In addition, an operation of collecting projection data of the number of views required to reconstruct one CT image is called a scan.
As a technique for improving the image quality of a reconstructed tomographic image (reconstructed image) while suppressing the amount of exposure of an object, there is a method of determining the tube current value using the image noise standard deviation value (image SD (Standard Deviation)) as an image quality index (for example, refer to PTL 1, PTL 2, and PTL 3). The image SD is a standard deviation of the CT value in the reconstructed image, and is influenced by tube current, the size (X-ray transmission length) of the object, and the like. PTL 1, PTL 2, and PTL 3 disclose the technique of controlling the amount of radiation by setting the target image SD and determining the tube current to realize the target image SD.
By using the technique disclosed in PTL 1, PTL 2, and PTL 3, it is possible to obtain an image that almost satisfies the target image SD. However, the visibility (diagnosability) of the diagnostic target (lesion) within the obtained tomographic image largely depends on the contrast of the image. For example, in a contrast examination using an X-ray CT apparatus, the contrast enhancement effect of tissue (for example, a blood vessel or digestive tract) by a contrast agent enables high-accuracy image diagnosis.
The contrast means an absolute value of the CT value difference between the lesion and the surrounding tissue, for example, and is a different concept from the image SD. Even if the image SD is adjusted to fall within a desired range, desired diagnosability is not necessarily obtained depending on to the size of the contrast. Therefore, there is a technique of determining the imaging conditions taking not only the image SD but also the contrast into consideration (for example, refer to PTL 4). PTL 4 discloses a technique of using a contrast-to-noise ratio (hereinafter, referred to as a “CNR”), which is obtained by dividing the contrast of a diagnostic target and its surroundings by the image SD, as an image quality index. In the technique disclosed in PTL 4, a tube current to achieve the CNR suitable for identifying a diagnostic target is determined on the basis of the size of the diagnostic target input by the operator and the assumed contrast.